1. Field of the Invention
The present invention relates to simulated three-dimensional audio and, more particularly, to head related transfer functions.
2. Description of the Relevant Art
Head related transfer functions (HRTFs) are used to simulate positional three-dimensional (3-D) sound using fixed speaker locations. The shape of the human head, body and auditory system affect how the brain perceives the position of sound sources. An HRTF is a characterization of the human head, body and auditory system. The HRTF primarily accounts for the frequency response, frequency filtering, delays and reflections inherent in the human head, body and auditory system. By adjusting the frequency and delays of audio signals according to the HRTF, three-dimensional sound can be simulated from fixed speaker locations.
The HRTF for each individual is unique. As mentioned above, the HRTF characterizes the human head, body and auditory system. The HRTF is affected by the size and shape of the head, the size and shape of the pinnae, the characteristics of the ear canal, and the relationship of the shoulder to the ear. A unique HRTF can be calculated for each individual by performing detailed and time consuming measurements of the head, ear and body. The measurements taken for an individual are converted to a transfer function usable by a processing device to adjust the characteristics of audio signal outputs to individual speakers to simulate positional three-dimensional sound.
The detailed measurements required to determine the HRTF of an individual are time consuming and require special purpose equipment. Determining the HRTF of an individual by taking measurements is suitable for low volume special purpose applications where accuracy is important and cost is relatively unimportant. Taking individual measurements to determine an HRTF, however, is not suitable for high volume applications in which cost is a primary concern, e.g., computer games. Computer games may use HRTFs to simulate positional 3-D sound. Because the HRTF is different for each individual, determining the HRTF for each individual user of a computer game would require making detailed time consuming measurements of each user using special purpose equipment. This, of course, is not practical for widely distributed computer games.
A common alternative to individualized HRTFs in high volume applications is the use of a generalized HRTF. A generalized HRTF is an average HRTF. The generalized HRTF is an attempt to define an HRTF that is effective for a large percentage of the population. The generalized HRTF works well for some portion of the population, works poorly for some percentage of the population and may not work at all for some portion of the population. Therefore, a general HRTF is a marginal solution to the problem of selecting an effective HRTF for high volume applications.
Another solution for determining individual HRTFs in high volume applications is to define a finite number of lesser-generalized HRTFs. Each lesser-generalized HRTF consists of various combinations of head, pinnae, and auditory canal characteristics. These HRTs are referred to as lesser generalized HRTs because they are an average HRTF for a subset of the general population. Each lesser-generalized HRTF is suited for some portion of the population and therefore the combination of lesser-generalized HRTFs provide increased accuracy and performance for a wide range of the population. Unfortunately, it is difficult to determine which of the HRTFs is the most appropriate, or the best fit, for an individual user. Measurements of the head, ear and body typically must be made to determine the most appropriate HRTF for each user. Although these measurements may be less detailed and time consuming than the measurements to define an individualized HRTF, the measurements to determine the most appropriate HRTF are too detailed and time consuming to be practical for high volume applications. Additionally, the measurements may require specialized equipment not readily available to individual users.
What is desired is a system and method for accurately selecting or adjusting an HRTF for an individual without requiring detailed measurements of the individual.